1. Field of the Invention
The present invention is directed to a pacemaker having the capability of detecting physical stress in a patient in whom the pacemaker is implanted in order to adjust the stimulation rate, as necessary dependent on the detected physical stress, as well as to a method for operating a pacemaker for achieving such adjustment.
2. Description of the Prior Art
A number of methods are known for adapting the stimulation rate of a heart pacemaker to the stress situation of the patient. A broad differentiation can be made between systems which use non-physiological parameters such as, for example, the movement (activity) of the body, and systems which use physiological parameters of the body such as, for example, blood pressure or blood temperature, for stimulation rate. Non-physiological rate-adaptive systems have the disadvantage that they can lead to stimulation rates which are not accurately matched to actual cardiac demand (upstairs-downstairs-paradox). Physiologically regulating sensors have the disadvantage that they require special sensors to measure suitable regulating parameters of the cardiac circulatory system, and these sensors do not have long-term stability. For this reason a number of processing augmentations, as well as non-specific measurement procedures, have to be carried out with standard catheters having long-term stability, such as intracardial impedance measurement for the heart pacemaker and defibrillator control. A significant feature of these known processing augmentations is the use of special methods in order to filter the signal content out of the polymorphic signal, this signal content in turn being clearly assigned to physiologically definable occurrences. A differentiation is made between two concepts:
1. Passive measurement of intracardial and intrathoracic measurement values without intentional influence of the measurement path by the stimulator, PA0 2. Active measurement of intracardial measurement parameters with defined influence of cardiovascular functions by the stimulator.
Active measuring systems, such as those operating through modulation of the stimulation rate of the heart muscle, have the advantage of suppressing non-cardiac disturbance by phase-synchronous analysis, and are preferred for the use of physiological function parameters for frequency regulation.
In the case of active measuring systems, it is known to use signal evaluation, related to frequency change, of action parameters of the heart to analyze intracardial impedance signals for rate regulation in heart pacemakers as described, for example, in U.S. Pat. No. 5,360,436. In this context, efforts have been made to evaluate the signal configuration of an intracardial measurement parameter during a heart pulse (n+1) dependent on frequency changes .DELTA.HR of a single preceding pulse (n). It has also been proposed to use the difference in the signal alterations of two successive pulses (n+1) and (n+2) dependent on the frequency of the first pulse (n). In order to standardize the individual or difference values, a quotient is calculated from the respective measurement value and a maximum change value determined at a higher frequency change. A substantial disadvantage of this arrangement is that this method is only successful with measurement signals at which an exact separation is possible between volume-caused and pressure-caused changes. This is, however, difficult in the case of conventional impedance measurement using a single-pole catheter because, in addition to the shape and volume of the ventricle, pressure changes at the electrodes significantly influence the signal configuration. In order to compensate for these influences it is necessary in setting up the system to carry out a separation of the pressure- and volume-dependent signal components. The disclosure of the aforementioned patent, however, gives no indication of how this separation is to be accomplished.
It is further known to optimize the rate regulation by means of periodic alteration of the stimulation rate, causing alterations in the cardiac volume or per given time unit to occur synchronously with the frequency alterations, with measurements thereof then being evaluated. This known procedure has the disadvantage that the time-constant of measurement of the cardiac volume with the required precision increases in such a way that optimization is only possible in long phases of uniform stress, i.e. no short-term adaptations to stress can be carried out (German OS 38 03 473).